The present invention relates to a method for manufacturing aluminum nitride with high yield using a one-step heating process of nitriding aluminum at temperatures below the melting point of aluminum at atmospheric pressure and aluminum nitride manufactured by the same.
Aluminum nitride possesses high thermal conductivity (320 W/m·K, ten times higher than that of alumina), high electrical insulation (9×1013 Ω·cm and low thermal expansion coefficient (4×10−6/° C.) which is close to that of silicon as well as other superior properties such as mechanical strength and chemical stability. For such reasons, aluminum nitride is widely used as thermal barrier materials, semiconductor and compound semiconductor substrates and as a reinforcing phase in composite materials. Recently, its use has been broadened to heat dissipating materials in LED devices because of its high thermal conductivity and low thermal expansion coefficient.
A variety of methods have been developed for the manufacturing of aluminum nitride up to now. Today, commercially available aluminum nitride is produced by either the Direct Nitridation Method or the Carbothermal Reduction Reaction. Direct Nitridation Method has the advantage of employing cheaper starting materials and producing aluminum nitride powders at a lower manufacturing temperature with a relatively simple process compared to the Carbothermal Reduction Reaction.
The direct nitridation reaction of aluminum powder occurs in accordance with the following scheme and is known to be thermodynamically possible at temperatures as low as room temperature and higher.Al(s)+½N2(g)=AlN(s)
It is worthy to note that the above reaction is highly exothermic and is accompanied by a considerable amount of heat. As a result, unreacted aluminum melted by the reaction heat coalesces and thus causes to impede further nitridation by blocking diffusion pathways for the supply of nitrogen gas. To prevent this, commercial direct nitridation methods employ prolonged heating at highly elevated temperatures of 1,000° C.˜2,000° C. for the complete nitridation of coalesced aluminum.
In addition, commercial direct nitridation methods repeatedly perform nitridation and pulverization of the aluminum nitride produced to increase overall yield or otherwise require further processing steps such as the addition of aluminum tri-fluoride (AlF3) or aluminum nitride (AlN) to facilitate the completion of the reaction. This could have negative side effects. During the pulverization process, for instance, the content of impurities such as oxygen may buildup and have a negative effect on thermal conductivity of the end product. In addition, the required additional processing steps may further increase the overall cost of manufacturing.
The direct nitridation method employs nitrogen or ammonium gas which reacts directly with aluminum powder to produce aluminum nitride. Accordingly, it is extremely difficult to control impurity contents to under several weight percent because unreacted aluminum is constantly being introduced as an impurity during the pulverization process as described in Korean Patent KR 10-1989-0002053 B1.
Direct nitridation methods typically use aluminum particles with a diameter less than 400 μm and conduct a nitridation treatment at temperatures from 900° C. to 1,400° C. before pulverizing and milling the aluminum nitride produced. In order to increase the degree of nitridation, crushed aluminum flakes are mixed together with aluminum nitride particles and then the mixed powder is used as a starting material. Another way to increase the degree of nitridation is to subject aluminum particles less than 250 μm in diameter to an initial nitridation treatment below the melting temperature of aluminum then pulverize the resulting aluminum nitride to an average particle size of 15 μm in diameter before subjecting the particles to an additional step of nitridation treatment at a temperature of 1,300° C.˜1,400° C. as described in Japanese Patent JP 61-083608 A. Still another way to increase the degree of nitridation is to add an ammonium compound containing fluorine and aluminum nitride powder to pure aluminum powder, then blending the mixture and heating it to a temperature of 430° C.˜650° C. for the initial nitridation treatment then repeating an additional step of nitridation at a temperature of 900° C.˜1,300° C. as according to Japanese Patent JP 62-003007 A.
European patent application EP 1,310,455 A1 discloses a process of manufacturing aluminum nitride in a specially designed furnace under nitrogen pressure falling in a range from 105 to 300 kPa for 30˜120 minutes before conducting nitridation at 500° C. to 1,000° C. It further discloses that fine aluminum nitride powder was obtained by supplying a reaction controller gas (argon or ammonia) into the reaction chamber to control the temperature at which nitridation took place whenever the temperature of the powder increased or nitrogen gas pressure in the reaction chamber decreased as the nitridation reaction proceeded.
As can be seen, up until now, known manufacturing methods of aluminum nitride using the Direct Nitridation Method feature heating for prolonged periods of time at temperatures much higher than the melting point of aluminum or heating at much lower temperatures but at higher pressures that require special equipment or multiple steps. Aluminum nitride particles exposed to such high temperatures tend to undergo self-sintering, which calls for additional high energy pulverizing processes to break them up, thus adding to overall cost. Likewise, a necessity for high pressure equipment adds to initial investment costs and employing multiple heating steps to increase yield impairs productivity since it requires additional operations and hence more time.
Since particle size of aluminum significantly influences an ensuing nitridation process it is very important to determine the particle size distribution of the initial aluminum particles involved in the process. In the present invention, the description of volume distribution of the particles is adopted using at least a median D50 value. Occasionally, though, to provide more detail, three values of size; D10, D50 and D90 are used. D50 is the median in which half of the population of particles lies below this value. Similarly 90 percent of the distribution lies below the value D90 and 10 percent of the particle population lies below the value D10. A three point specification as such is considered complete and more than appropriate for describing the size of powder materials involved for the purpose of explaining the present invention.
The present invention enables manufacturing aluminum nitride with a high yield by way of a single-step heating process at a temperature which is lower than the melting point of aluminum at atmospheric pressure in a relatively short amount of time.